The present invention relates to an elastic surface wave device wherein the absorbers used to attenuate unwanted elastic waves are more effective, and the process for forming said absorbers is simple.
In elastic surface wave devices, the reflection of elastic surface waves from the substrate edges on the extensions in the elastic surface wave propagation path generates ripples in the amplitude frequency and phase frequency characteristics. These unwanted reflected waves must be prevented from occurring. In the past, elastic surface wave absorbers have been placed near the substrate edges on the extensions in the elastic surface wave propagation path.
FIG. 1 is a plan view of a surface elastic wave device provided with said absorbers. Here, 1 represents a piezoelectric substrate, 2 an input electrode that converts an electric signal to an elastic surface wave, 3 an output electrode that converts an elastic surface wave into an electrical signal, 4 and 4' elastic surface wave absorbers that attenuate or extinguish the elastic surface waves, and 5 and 5' substrate edges. Half the energy of the elastic surface waves excited by input electrode 2 travels directly towards output electrode 3, while the remaining half proceeds in the reverse direction, is reflected at substrate edge 5' on the input electrode side, and eventually reaches output electrode 3. Nor are all the elastic surface waves that initially proceed towards output electrode 3 converted directly into electrical signals at the output electrode; half of the elastic surface wave energy moving in this direction travels to substrate edge 5 on the output electrode side, where it is reflected back again to output electrode 3. The reflected waves generated at these edges become unwanted signals having a time lag with respect to the main signal, resulting in the appearance of ripples in the aforementioned frequency characteristics. Elastic surface wave absorbers 4 and 4' are provided to fully attenuate or extinguish the unwanted waves by first attenuating the elastic surface waves while transmitting them to edges 5 and 5', then once again attenuating the reflected waves proceeding inwards from these edges; in this way, they prevent adverse effects by the unwanted waves upon the main signal.
In conventional elastic surface wave devices, thermosetting resins such as epoxy resin are used to form the elastic surface wave absorbers. For example, Japanese Patent Application Kokai (Laid-open) No. 56-36815 mentions that, based on comparisons of a number of different materials, epoxy resins show excellent elastic surface wave attenuation characteristics. It also notes that these attenuation characteristics are further improved by mixing and dispersing powders such as cement, nickel, silver, Al.sub.2 O.sub.3, Bi.sub.2 O.sub.3, WO.sub.3, or the like in the epoxy resin as fillers.
However, epoxy resins have the following two disadvantages.
The first of these is that the reflection at the edge of epoxy resin elastic surface wave absorbers proper is considerable, remaining greater than -40 dB with respect to the main signal, i.e. surface acoustic wave. In experiments conducted by the present inventors, the results obtained ranged from about -25 to -35 dB. This value of -40 dB or less with respect to the main signal is a standard that has been set for unwanted signal wave levels in light of the ghost detection limit in applications to elastic surface wave filters for TV-IF (Intermediate Frequency Circuit of Television Receiver), and the like, (refer to, for example, pp 504-508 of the 1978 IEEE Transactions on Sonics and Ultrasonics). Measures such as those presented, for example, in U.S. Pat. No. 4,188,596, Japanese Patent Application Kokai (Laid-open) No. 53-109458, and Japanese Patent Application Kokai (Laid-open) No. 54-4054 have been taken to meet this standard. These consist, for instance, of scattering the unwanted waves by providing epoxy resin elastic surface wave absorbers with a sawtoothed pattern, as seen in vertical cross-section, or of applying silicon resin or some like material that has little attenuation but generates only limited reflection at the absorber edges either adjacent to the edges, or on top, of the epoxy resin, etc. However, this increases the size of the substrate in the elastic surface wave device. In addition, because two types of absorbers are included in the device, the number of manufacturing steps increases, driving up the cost of production. This drawback is especially telling when the substrate dimensions are increased in order to reduce the working frequency, increase the wavelength of the elastic surface waves, and expand the width of the elastic surface wave absorbers in the direction of propagation.
The second disadvantage of epoxy resins is that, because these are thermosetting resins, they have a long curing time; as a result, it is difficult to automate the production steps, making such resins unsuitable to mass production. In addition, this invites numerous other problems responsible for poor mass production potential and inflated production costs, such as cracking and increased crackability of the piezoelectric substrate under heating, and also poor curing as a result of fluctuations in the curing temperature.